1. Field of the Invention
This invention relates to a picture signal processing method and apparatus. More particularly, it relates to a picture signal processing method and apparatus in which, in compression encoding and decoding input data, such as still picture data or moving picture data, using block encoding of blocking and DCTing the input data, distortion or noise can be removed effectively.
2. Description of Related Art
As an encoding system for efficiently compression-encoding still picture data or moving picture data, there is known block encoding such as block DCT (discrete cosine transform) encoding.
In compressing/expanding picture data by this block encoding, block distortion (block noise) tends to be produced, such that, the higher the compression factor, the larger becomes the distortion. This block distortion is produced by the fact that DCT is executed in a closed space in a block without taking account of the correlation beyond the block boundary. Thus, continuity cannot be maintained in the block boundaries, so that the deviation in the playback data value at the boundary area to the neighboring blocks is perceived as noise. The block distortion, produced on block encoding picture data, exhibits certain periodicity and hence is perceived more readily than the usual random noise so that it proves to be a significant factor in producing picture quality deterioration.
For reducing this block distortion, there is disclosed in Ida and Datake "Noise Removing Filter in MC-DCT Encoding System", Extended Abstract to Spring Meeting of Society of Electronic Information Association, 1990, 7-35, a technique of employing the quantization step size for setting the filter on/off or of performing processing a plural number of times under varying the proceeding direction for preserving the edge as the information proper to a picture for removing the noise. There is also disclosed in Izawa, "Characteristics of Adaptive Noise Removing Filter in Picture Block: Encoding", a report by Technical Department of Shinshu University, No.74, pp.89-100, a technique of extracting even ambient blocks to execute DCT to remove noise frequency components.
With the former method, processing is simpler, however, the high-frequency components in a picture are lost. This point needs to be corrected, as in the latter method, so that there will be no fear of loss of the high frequency components.
As a method for removing the block distortion or mosquito noise, produced on picture compression/expansion thereafter compression, there is proposed an algorithm of using prediction of local statistic quantities, such as picture flatness, and the block-based noise quantities from the coding information, pixel-based noise prediction and adaptive filtering by the least square method, for removing the noise.
The mosquito noise means a ringing output picture distortion produced by high frequency components in the quantization error.
Meanwhile, it is known to correct an input picture by picture quality correction, such as by contour enhancement, for improving the picture definition. If picture quality correction, such as contour enhancement, is effected at the time of picture compression/expansion accompanying block encoding, the above-mentioned block distortion or mosquito noise is emphasized to render it difficult to improve the picture quality sufficiently.
In e.g., a so-called video CD player, there is proposed a structure in which the noise is removed by a field recursive type noise filter, periodic signals are appended by a NTSC encoder, the resulting signal is converted by a D/A converter into analog signals and the resulting analog signals are processed by a contour enhancement circuit for improving the picture definition.
However, in this case, the block distortion or the mosquito noise, not removed by a field recursive type noise reducer, is also amplified by the contour enhancement circuit to render it difficult to achieve sufficient improvement in the picture quality.